1. Field of the Invention
The present invention relates to the use of hydroxyl-functional polyalkylorganosiloxanes as solvents for cationic photoinitiators and compositions comprising cationic photoinitiators, to hydroxyl-functional polyalkylorganosiloxanes as solvents and to their use in cationically radiation-curable silicones.
2. Description of Related Art
Since the 1980s cationically radiation-curing abhesive coating materials have been known on the market. They are composed of organopolysiloxanes which contain reactive groups and which cure under UV radiation by a cationic mechanism. Said reactive groups may be epoxy groups, vinyl ether groups or alkenyloxy groups such as vinyloxy groups or propenyloxy groups. Such substances are described for example in U.S. Pat. Nos. 5,057,549; 5,231,157; 4,421,904; 4,547,431; 4,952,657; 5,217,805; 5,279,860; 5,340,898; 5,360,833; 5,650,453; 5,866,261 and 5,973,020.
Cationic photopolymerization is a rapid, efficient and environmentally benign way to cure cationically polymerizable monomers. Particularly efficient photoinitiators are diaryliodonium salts (I) and triarylsulfonium salts (II)Ar—I+—Ar X−  (I)
X−=BF4−, PF6−, AsF6−, SbF6−
where Ar represents identical or different aromatic radicals which if desired may contain heteroatoms and/or further substituent radicals.
Diaryliodonium salts (I) in particular are known from the patent literature (GB-A-1 516 352, U.S. Pat. No. 4,279,717, EP-A-0 334 056, U.S. Pat. No. 5,468,890) and are used as photoinitiators for polymerizing cationically polymerizable substances.
One important requirement for the utility of the photoinitiators is their miscibility in the cationically radiation-curable organopolysiloxanes. In order to ensure such miscibility the photoinitiators are prepared or modified with a hydrophobic radical, as described for example in U.S. Pat. Nos. 4,310,469; 6,365,643 and 6,380,277.
In pure form, however, such photoinitiators are solids or at least waxlike or viscous compounds and hence are difficult to incorporate into cationically radiation-curable organopolysiloxanes. For example the commercially available cationic photoinitiator bis(dodecylphenyl)iodonium hexafluoroantimonate (III),
has a waxlike consistency at room temperature.
In order to ensure ease of incorporation of such photoinitiators into the cationically radiation-curable compositions, the photoinitiators are generally supplied in dissolved form. Used for this purpose are solvents such as toluene, xylene, isopropanol, propylene glycol, alkylphenols, propylene carbonate, glycidyl ethers and butyrolactone.
One important requirement for the utility of such solvents is that the cationic photoinitiators form clear solutions therein and form clear mixtures with the cationically radiation-curable organopolysiloxanes. With propylene carbonate, ethylene glycol or propylene glycol, for example, it is the case, however, that the frequently very hydrophobic cationically curable organopolysiloxanes are generally unable to form clear mixtures. Other solvents such as C12/C14 glycidyl ethers, for example, are environmentally hazardous.
Low molecular mass, highly volatile compounds in particular are of only limited suitability as solvents for the cationic photoinitiators, since they burden the environment by odors and/or volatile solvent constituents (VOCs, volatile organic compounds) and harbor the risk of formation of explosive gas mixtures.
In order to prevent the environment being burdened by odors and/or VOCs the aim is to use high-boiling solvents which additionally by way of functional groups can be incorporated into the cationically curable coating and hence do not remain as a migrateable constituent in the cured silicone release coating. High-boiling solvents of this kind may be, for example, alkylphenols or alkyl epoxides, as described in EP-A-0 522 703.
Particularly in the case of coatings produced from organopolysiloxanes containing epoxy groups, however, it is observed that polar solvents, such as isopropanol, propylene glycol and alkylphenols, may adversely affect the release activity of the organopolysiloxane coating. U.S. Pat. No. 4,954,364 describes for example the addition of alkylphenols to organopolysiloxanes containing epoxy groups as a controlled release additive (CRA) in order deliberately to detract from the release property of the organopolysiloxane coating. The addition of just 0.1 part of nonylphenol to the organopolysiloxane containing epoxy groups produces a doubling in the release force as compared with a nonylphenol-free coating. The release force (or release value) is the force required to peel the respective adhesive tape from the substrate. Generally speaking, however, the aim when using an organopolysiloxane coating is to obtain a very good release effect, i.e., low release force.
It is therefore desirable to find high-boiling solvents which do not burden the environment with odors and/or volatile solvent constituents (VOCs, volatile organic compounds) and which do not adversely affect the release activity of organopolysiloxane coatings.
A further important requirement for the utility of such solvents is that the mixtures of solvent and photoinitiator exhibit at least six months' storage stability. It is desirable, in other words, to find solvents which form clear solutions with the cationic photoinitiator and which have a storage stability of at least six months.
For the preparation of cationic photoinitiators, as described in applications U.S. Pat. Nos. 6,365,643 and 6,380,277, the substances used are in some cases toxic, corrosive and/or environmentally hazardous, such as isocyanates, acid chlorides or chlorosilanes, for example. For reasons of the reaction process these substances are used in excess, an excess which it is then impossible to remove sufficiently by distillation afterward, owing to the thermal lability of the iodonium salts.
It is therefore also desirable to find solvents which together with the remnant excesses are consumed by reaction to form nonhazardous, polymeric substances.